Method for manufacturing in-plane lattice constant adjusting substrate and in-plane lattice constant adjusting substrate

ABSTRACT

A method of adjusting the in-plane lattice constant of a substrate and an in-plane lattice constant adjusted substrate are provided. A crystalline substrate ( 1 ) made of SrTiO 3  is formed at a first preestablished temperature thereon with a first epitaxial thin film ( 2 ) made of a first material, e. g., BaTiO 3 , and then on the first epitaxial thin film ( 2 ) with a second epitaxial thin film ( 6 ) made of a second material, e. g., BaxSr 1−x TiO 3  (where 0&lt;x&lt;1), that contains a substance of the first material and another substance which together therewith is capable of forming a solid solution in a preestablished component ratio. Thereafter, the substrate is heat-treated at a second preselected temperature. Heat treated at the second preestablished temperature, the substrate has dislocations ( 4 ) introduced therein and the second epitaxial thin film ( 6 ) has its lattice constant relaxed to a value close to the lattice constant of bulk crystal of the second material. Selecting the ratio of components x of the other substance in the second material allows a desired in-plane lattice constant to be realized.

TECHNICAL FIELD

The present invention relates to a method of preparing an in-planelattice constant adjusted substrate, and an in-plane lattice constantadjusted substrate, whereby a desired in-plane lattice constant can berealized.

BACKGROUND ART

A thin film formed so that it is epitaxially grown on a crystallinesubstrate has its properties influenced by its crystalline perfectness.For example, in the preparation of an oxide superconductor thin filmsuch as of (Ba, Sr) CuO type which is epitaxially grown on a substrateto form a laminated superconductor, its superconductive transitiontemperature and superconductive critical magnetic field are influencedby its crystalline perfectness such as its crystal defect density andcrystallographic orientation. Also, an epitaxial BaTiO₃ thin film usedas a memory element in a semiconductor integrated circuit has itscapacity value largely varied by its crystallographic orientation.

So far, in order to obtain a high quality thin film that is satisfactoryin crystalline perfectness, a substrate has been used having an in-planelattice constant that is close to the in-plane lattice constant of thethin film. If there does not exist any substrate having an in-planelattice constant close to that of a thin film, a material is chosenhaving an in-plane lattice constant intermediate between those of thesubstrate and the thin film and is layered on the substrate as a bufferlayer on which the thin film may be grown.

It is, however, only rare that a substrate is available that agrees inin-plane lattice constant with the thin film, and if such a substrate isavailable, it often is extremely brittle or of high cost. In the use ofa buffer layer, too, it rarely is the case that a substrate is availablewhich is fully congruous in in-plane lattice constant.

Thus, in the past, since it has not been possible to grow a thin film ona substrate that fully agrees or is congruent in in-plane latticeconstant therewith, it has been likely the case that a thin film hasdislocations introduced therein due to its lattice mismatch with thesubstrate; hence a thin film results that is highly dense with crystaldefects, imperfect in crystalline orientation, and thus poor in qualityand properties, problems met by the prior art.

DISCLOSURE OF THE INVENTION

In view of the aforementioned problems in the prior art, the presentinvention has for its object to provide a method of adjusting thelattice constant of a substrate and to provide an in-plane latticeconstant adjusted substrate.

In order to achieve the object mentioned above, there is provided inaccordance with the present invention in a first form of implementationthereof a method of preparing an in-plane lattice constant adjustedsubstrate, characterized in that it comprises the steps of: growing afirst epitaxial thin film made of a first material on a substrate at afirst preestablished temperature; and heat-treating at a secondpreestablished temperature the substrate having the first epitaxial thinfilm grown thereon.

In the method of preparing an in-plane lattice constant adjustedsubstrate in accordance with the present invention, the said firstpreestablished temperature is a temperature that causes the said firstmaterial to epitaxially grow on the said substrate.

Also, the said second preestablished temperature is a temperature thatis higher than the said first preestablished temperature but lower thanthe lower of melting points of the said substrate and the said firstepitaxial thin film.

According to this method makeup, epitaxially growing a first material ona substrate at a first preestablished temperature causes a firstepitaxial thin film having distortions due to its mismatch in in-planelattice constant with the substrate to epitaxially grow on thesubstrate, and heat-treating the epitaxial thin film and the substrateat a second preestablished temperature introduces dislocations into thesubstrate surface and relaxes the in-plane lattice constant of the firstepitaxial thin film to a value close to the lattice constant of bulkcrystal of the first material. The dislocations are anchored to theinterface between the substrate and the first epitaxial thin film, thetop surface of the epitaxial thin film is flattened to an atomic level,and they are left immobile when another material is epitaxially grown onthat surface.

In the present invention, the said substrate and the said firstepitaxial thin film are preferably made of oxides. It is also preferredthat the said substrate be made of SrTiO₃ crystal and said firstepitaxial thin film be made of BaTiO₃.

According to this method makeup, it is possible to adjust the in-planelattice constant of a single crystal substrate of SrTiO₃ which is of lowcost and stout, to the lattice constant of BaTiO₃. Also in anapplication in which the in-plane lattice constant of BaTiO₃ isrequired, it can be used in place of a BaTiO₃ which is brittle and ofhigh cost. For example, it becomes possible to manufacture at a low costBaTiO₃ capacitors which are high in dielectric constant.

The present invention also provides in another form of implementationthereof a method of preparing an in-plane lattice constant adjustedsubstrate, characterized in that it comprises the steps of: forming on asingle crystal substrate whose surface is flat on an atomic level, afirst epitaxial thin film having a first preselected film thickness andmade of a first material that is different from a material of which thesubstrate is made, and then forming on the first epitaxial thin film, asecond epitaxial thin film having a second preselected film thicknessand made of a second material that contains, at a predetermined ratio ofcomponents, a substance of said first material and another substancewhich is capable of forming a solid solution; and thereafterheat-treating them at a second preestablished temperature that is higherthan an epitaxial growth temperature of the said first and secondepitaxial thin films but lower than the lowest of melting points of thesaid substrate, the said first epitaxial thin film and the said secondepitaxial thin film to introduce dislocations into an interface betweenthe said substrate and the said first epitaxial thin film and aninterface between the said first and second epitaxial thin films,whereby a modification of the said substrate ensures having an in-planelattice constant of the said second epitaxial thin film controllablydetermined by a ratio of the said first to second film thickness and/ora said predetermined ratio of components and having the top surface ofthe said second epitaxial thin film flattened to an atomic level.

Also, the said second epitaxial thin film may be formed to the secondpreselected film thickness on the said substrate without using the saidfirst epitaxial thin film made of the first material and may thereafterbe heat-treated at the second preestablished temperature.

According to this method makeup, on the substrate there is allowed toepitaxially grow a first epitaxial thin film having distortions due toits mismatch in in-plane lattice constant with the substrate, and on thefirst epitaxial thin film there is allowed to grow epitaxially a secondepitaxial thin film having distortions due to its mismatch in in-planelattice constant with the first epitaxial thin film, and thenheat-treating them at a second preestablished temperature introducesdislocations into the substrate surface and into the interface betweenthe first and second epitaxial thin films and relaxes the in-planelattice constants of the first and second epitaxial thin films to valuesclose to the lattice constant of the bulk crystal of the secondmaterial.

The dislocations are anchored to the interfaces between the substrateand the first epitaxial thin film and between the latter and the secondepitaxial thin film, the top surface of the second epitaxial thin filmis flattened to an atomic level, and the dislocations are left immobilewhen another material is caused to grow epitaxially on that surface.

Suitably selecting the ratio of components of the other substance in thesaid second material allows a desired in-plane lattice constantdetermined by the selected ratio of components and hence a substratehaving such a desired in-plane lattice constant to be obtained.

Further, the said substrate and the said first and second epitaxial thinfilms are preferably made of oxides. It is preferred that the saidsubstrate be a SrTiO₃ crystalline substrate, the said first epitaxialthin film be made of BaTiO₃ and the said second epitaxial thin film bemade of Ba_(x)Sr_(1−x)TiO₃ where 0<x<1.

According to this method makeup, suitably selecting x allows forming asubstrate which agrees in in-plane lattice constant with a thin film tobe formed thereon. Thus, for example, such a substrate can be used forforming a (Ba, Sr) CuO or like oxide superconductor thin film thereon.Then, since a substrate can be formed which by selecting x suitably ismade substantially identical in in-plane lattice constant to asuperconductor layer to be formed thereon, it becomes possible to obtainan oxide high-temperature superconductor film that extremely excels inquality. It is also possible to adjust the in-plane lattice constant ofthe second epitaxial thin film by adjusting the ratio in film thicknessof the first to second epitaxial thin film.

The present invention also provides an in-plane lattice constantadjusted substrate, characterized in that it comprises a crystallinesubstrate made of SrTiO₃ and having a thin film of BaTiO₃ formedthereon, wherein the BaTiO₃ thin film has its top surface flattened toan atomic level and is substantially equal in lattice constant to BaTiO₃bulk crystal. A substrate so made up may be used, in an application inwhich a BaTiO₃ substrate is required, to replace the same which isbrittle and of high cost.

The present invention also provides a in-plane lattice constant adjustedsubstrate, characterized in that it comprises a crystalline substratemade of SrTiO₃ and having a thin film of BaTiO₃ formed thereon and athin film of Ba_(x)Sr_(1−x) TiO₃ (where 0<x<1) formed on the BaTiO₃ thinfilm, wherein the Ba_(x)Sr_(1−x) TiO₃ thin film has its top surfaceflattened to an atomic level and has its lattice constant adjustable toa desired length between the lattice constants of SrTiO₃ and BaTiO₃ bulkcrystals by selecting x. A substrate so made up can be used in forming athin film thereon. Then, since a substrate can be formed that agrees inin-plane lattice constant to a thin film to be formed thereon, it ispossible to form a thin film that extremely excels in quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will better be understood from the followingdetailed description and the drawings attached hereto showing certainillustrative forms of implementation of the present invention. In thisconnection, it should be noted that such forms of implementationillustrated in the accompanying drawings hereof are intended in no wayto limit the present invention but to facilitate an explanation andunderstanding thereof. In the drawings:

FIG. 1 shows typical views illustrating the principles of preparing anin-plane lattice constant adjusted substrate according to a first formof implementation of the present invention;

FIG. 2 shows typical views illustrating the principles of preparing anin-plane lattice constant adjusted substrate according to a second formof implementation of the present invention;

FIG. 3 shows images by a scanning tunnel electron microscope of asubstrate surface, an epitaxial thin film surface before a heattreatment and the epitaxial thin film surface after the heat treatmentin the first form of implementation of the present invention;

FIG. 4 shows an image taken by an atomic force microscope (AFM) of anin-plane lattice constant adjusted substrate surface according to thefirst form of implementation of the present invention;

FIG. 5 shows images of a specimen comprising a SrTiO₃ substrate with aBaTiO₃ thin film epitaxially grown thereon in the first form ofimplementation of the present invention, which is taken by atransmission electron diffraction microscope (TEM) in a directionperpendicular to the substrate surface, wherein FIG. 5( a) is an TEMimage where spots therein correspond to lattice points and FIG. 5( b) isa figure obtained when the image of FIG. 5( a) is processed so as tovisualize the continuity of lattice planes;

FIG. 6 shows images of a specimen comprising a SrTiO₃ substrate having aBaTiO₃ thin film epitaxially grown on it and thereafter heat-treated inthe first form of implementation of the present invention, which istaken by the transmission electron diffraction microscope (TEM) in adirection perpendicular to the substrate surface, wherein FIG. 6( a) isan TEM image where spots therein correspond to lattice points and FIG.6( b) is a figure obtained when the image of FIG. 6( a) is processed soas to visualize the continuity of lattice planes;

FIG. 7 is a chart illustrating a result of measurement by a four-axisX-ray diffraction apparatus of a distribution of lattice constants in anin-plane lattice constant adjusted substrate according to the first formof implementation of the present invention;

FIG. 8 shows an image by the atomic force microscope (AFM) of thesurface of an in-plane lattice constant adjusted substrate according tothe second form of implementation of the present invention; and

FIG. 9 shows an image by the four-axis X-ray diffraction apparatus of adistribution of lattice constants in an in-plane lattice constantadjusted substrate according to the second form of implementation of thepresent invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail withreference to certain suitable forms of implementation thereofillustrated in the drawing figures.

At the outset, mention is made of a first form of implementation of thepresent invention.

FIG. 1 shows typical views illustrating the principles of preparing anin-plane lattice constant adjusted substrate according to a first formof implementation of the present invention wherein a substrate 1 has afirst epitaxial thin film 2 of a first material formed thereon. FIG. 1(a) shows a state in crystal lattice of the substrate 1 and the firstepitaxial thin film 2 typically in a cross section taken perpendicularto a surface 3 of the substrate. The crystal lattices in the substrate 1and those in the first epitaxial thin film 2 are typically representedby squares and rectangles, respectively, as shown. FIG. 1( b) shows astate of the crystal lattice after the heat-treatment of the substrate 1formed with the first epitaxial thin film 2 on it, which are showntypically in a cross section taken perpendicular to the substratesurface 3. The crystal lattices in the substrate 1 and those in thefirst epitaxial thin film 2 after the heat treatment are again typicallyrepresented by squares and rectangles, respectively, as shown.

The method of preparing an in-plane crystal lattice adjusting substratecomprises a first step of supplying the substrate 1 at a firstpreestablished temperature with a first material over it to grow a firstepitaxial thin film 2 composed of the first material on the substrate 1.The first preestablished temperature may be any low or high temperatureif it allows the epitaxial growth of the first epitaxial thin film 2.Here, any epitaxial growth process, such as MOCVD, CVD or laserablation, may be adopted in epitaxially growing the first material onthe substrate 1.

As shown in FIG. 1( a), the first epitaxial thin film 2 is formed on thesubstrate so that the lattice planes in the first epitaxial thin film 2are substantially continuous with those in the substrate 1 and that thein-plane lattice constant a₁ in the first epitaxial thin film 2 issubstantially equal to the in-plane lattice constant as in the substrate1. Also the lattice constant of the first epitaxial thin film 2 in adirection perpendicular to the substrate surface 3 is different from thelattice constant of the first material in its bulk state. Therefore thefirst epitaxial thin film 2 is grown on the substrate 1 in a state thatit has distortions due to its mismatch in in-plane lattice constant withthe substrate 1.

Then, the substrate 1 on which the first epitaxial thin film 2 has beengrown is heat-treated at a second preestablished temperature, which is atemperature higher than the first preestablished temperature, but lowerthan the lower of the melting points of the substrate 1 and the firstepitaxial thin film 2.

With this heat treatment, as shown in FIG. 1( b), a dislocation 4 isintroduced into the substrate surface 3 while the distortions in thefirst epitaxial thin film 2 is relieved with the result that thein-plane lattice constant a₁ in the first epitaxial thin film 2 becomessubstantially equal to the lattice constant of the first material in itsbulk state. Further, its lattice constant in a direction perpendicularto the substrate surface 3 also relaxes and becomes substantially equalto the lattice constant in its bulk state. The dislocations 4 areanchored to the substrate surface 3 while the first epitaxial thin film2 has its top surface 5 flattened to an atomic level, thereby permittinganother material to grow epitaxially thereon while leaving thedislocations 4 anchored immobile.

Mention is next made of a second form of implementation of the presentinvention.

FIG. 2 shows typical views illustrating the principles of preparing anin-plane lattice constant adjusted substrate according to a second formof implementation of the present invention, wherein a substrate 1 has afirst epitaxial thin film 2 of a first material formed thereon by apreselected film thickness, which in turn has a second epitaxial thinfilm 6 of a second material formed thereon by a predetermined thickness.FIG. 2( a) shows a state in crystal lattice of the substrate 1, thefirst epitaxial thin film 2 and the second epitaxial thin film 6typically in a cross section taken perpendicular to a surface 3 of thesubstrate 1. The crystal lattices of the substrate 1 and the crystallattices of the first and second epitaxial thin films 2 and 6 aretypically represented by squares and rectangles, respectively, as shown.

FIG. 2( b) shows a state in crystal lattice of the substrate 1 and thefirst and second epitaxial thin films 2 and 6 which are formed asmentioned above and then are heat-treated at a second preestablishedtemperature. The state is shown typically in a cross section takenperpendicular to the substrate surface 3. The crystal lattices of thesubstrate 1 and those of the first and second epitaxial thin films 2 and6 are again typically represented by squares and rectangles,respectively, as shown.

The method of a preparing an in-plane lattice constant adjustedsubstrate of the present invention in this form of implementationcomprises a first step of supplying a substrate 1 at a firstpreestablished temperature with a first material over it to grow a firstepitaxial thin film 2 of a first predetermined film thickness on thesubstrate 1 and subsequently supplying a second material to grow asecond epitaxial thin film 6 of a second predetermined film thicknessthereon. Here, the first predetermined film thickness is madesufficiently thinner than the second predetermined film thickness. Thesecond material should contain a substance of the first material andanother substance that is capable of forming a solid solution togetherwith that substance, at a preselected ratio of components. The firstpreestablished temperature may be any low or high temperature if itallows the epitaxial growth of the thin film. Here, any epitaxial growthprocess, such as MOCVD, CVD or laser ablation, may be adopted inepitaxially growing the first and second materials on the substrate 1.

As shown in FIG. 2( a), the first and second epitaxial thin films areformed on the substrate so that the lattice planes in the first andsecond epitaxial thin films 2 and 6 are substantially continuousmutually and with those in the substrate 1 and that the in-plane latticeconstants a₁ and a₂ in the first and the second epitaxial thin films 2and 6 are substantially equal to the in-plane lattice constant a_(s) inthe substrate 1. Therefore, the first and second epitaxial thin films 2and 6 are epitaxially grown on the substrate 1 in a state that they havedistortions due to their mismatch in in-plane lattice constant.

Then, the substrate 1 on which the epitaxial thin films 2 and 6 havebeen grown is heat-treated at a second preestablished temperature whichis a temperature higher than the first preestablished temperature, butlower than the lowest of the melting points of the substrate 1 and thefirst and second epitaxial thin films 2 and 6. With this heat treatment,as shown in FIG. 2( b), dislocations 4 are introduced into both thesubstrate surface 3 and the interface 7 between the first and secondepitaxial thin films 2 and 6 or the substrate surface 3 alone to relaxthe in-plane lattice constants in the first and second epitaxial thinfilms 2 and 6. Further, with the first predetermined film thickness madesufficiently thinner than the second predetermined film thickness, thein-plane lattice constants of the first and second epitaxial thin films2 and 6 relax to a value close to the lattice constant of the secondmaterial in its bulk crystal state. The in-plane lattice constant a₂changes according to the component ratio of the second material, namelythe ratio in amount of the substance of the first material to the othersubstance forming the solid solution together therewith. Their latticeconstants in a direction perpendicular to the substrate surface 3 alsorelax. The dislocations 4 are anchored to both the substrate surface 3and the interface 7 or to the substrate surface 3 alone while the secondepitaxial thin film 6 has its top surface 8 flattened to an atomiclevel, thereby permitting another material to grow epitaxially thereonwhile leaving the dislocations anchored immobile.

The present invention thus enables the in-plane lattice constant of asubstrate to be adjusted to a desired value, thereby permitting a devicepreparation process using an epitaxial thin film to be furnished with asubstrate having an optimum in-plate lattice constant.

Mention is next made of a specific example of the first form ofimplementation of the present invention.

A specific example is here shown in which a substrate of SrTiO₃ crystalis adjusted to have a in-plane lattice constant of BaTiO₃. As a specimenthe SrTiO₃ crystal substrate was formed thereon at an epitaxial growthtemperature of 650° C. with an epitaxial thin film of BaTiO₃ as thefirst epitaxial thin film to a film thickness of 120 angstroms by laserablation in a vacuum chamber, and then in the same vacuum chamber washeat-treated to a temperature of about 1350° C. for a period of about 1hour by laser heating.

FIG. 3 shows images by a scanning tunnel electron microscope of asubstrate surface, an epitaxial thin film surface before the heattreatment and an epitaxial thin film surface after the heat treatment.The BaTiO₃ epitaxial thin film grown on the SrTiO₃ crystal substratewhose surface is flat on an atomic level as shown in FIG. 3( a) is seento have a top surface that is keenly irregular or uneven as shown inFIG. 3( b). With the heat treatment applied to the substrate, a film topsurface that is flat or even on an atomic level as shown in FIG. 3( c)has obtained. Each of vertical stripes in FIGS. 3( a) and 3(c)represents an atomic surface step which corresponds to mono-molecularlayer, and a region between the adjacent vertical stripes represents anidentical atomic surface. FIG. 4 shows an image taken by an atomic forcemicroscope (AFM) of the surface of the specimen after the heattreatment. Each of vertical stripes seen in the figure represents atomicsurface step which corresponds to mono-molecular layer, and a regionbetween the adjacent vertical stripes represents an identical atomicsurface. From FIGS. 3 and 4, it is seen that the top surface of theBaTiO₃ epitaxial thin film is flat and even on an atomic level.

FIG. 5 shows images of a specimen comprising a SrTiO₃ substrate with aBaTiO₃ thin film epitaxially grown thereon, which is taken by atransmission electron diffraction microscope (TEM) in a directionperpendicular to the substrate surface, wherein FIG. 5( a) is an TEMimage where spots therein correspond to lattice points and FIG. 5( b) isa figure obtained when the image of FIG. 5( a) is processed so as tovisualize the continuity of lattice planes. From FIG. 5 it is seen thatthe substrate and the epitaxial thin film are almost continuous witheach other in lattice planes and that the epitaxial thin film has alarge number of discontinuities of lattice planes, namely dislocationstherein.

FIG. 6 shows images of a specimen comprising a SrTiO₃ substrate having aBaTiO₃ thin film epitaxially grown on it and thereafter heat-treated,which is taken by the transmission electron diffraction microscope (TEM)in a direction perpendicular to the substrate surface, wherein FIG. 6(a) is an TEM image where spots therein correspond to lattice points andFIG. 6( b) is a figure obtained when the image of FIG. 6( a) isprocessed so as to visualize the continuity of lattice planes. In FIG.6( a) it is seen that lattice points lie more orderly than in FIG. 5(a). From FIG. 6( b) it is seen that dislocations exist only on thesubstrate surface, and not at all on epitaxial thin film. From theseresults it is seen that according to the method of the present inventionthe dislocations are anchored to the substrate surface and the topsurface of the first epitaxial thin film is flattened to an atomiclevel.

FIG. 7 is a chart illustrating a result of measurement by a four-axisX-ray diffraction apparatus of a distribution of lattice constants inthe specimen mentioned above. In the chart, the abscissa and ordinateaxes are taken in the (300) and (003) directions of a reciprocal latticespace, respectively. In the chart, the point A indicates the diffractionpoint of the BaTiO₃ epitaxial thin film where its diffraction intensityis the maximum, and the point B indicates the diffraction point of theSrTiO₃ crystal substrate where its diffraction intensity is the maximum.From the coordinate of point A it is seen that the in-plane latticeconstant a of the BaTiO₃ epitaxial thin film is 3.990 angstroms, whichis approximately equal to the lattice constant of BaTiO₃ bulk crystal(a=4.000 angstroms). It is seen that according to the method of thepresent invention, the in-plane lattice constant of the first epitaxialthin film is relaxed to the lattice constant of its bulk crystal.

Mention is next made of a specific example of the second form ofimplementation of the present invention. Here, a specific example isshown in which a substrate of SrTiO₃ crystal is adjusted to have adesired in-plane lattice constant. As a specimen the SrTiO₃ crystalsubstrate was formed thereon at an epitaxial growth temperature of 650°C. with an epitaxial thin film of BaTiO₃ as a first epitaxial thin filmto a film thickness of 120 angstroms and then formed thereon at anepitaxial growth temperature of 650° C. with an epitaxial thin film ofBa_(0.5)Sr_(0.5)TiO₃ as a second epitaxial thin film to a film thicknessof 1800 angstroms, each by laser ablation in a vacuum chamber, and thenin the same vacuum chamber was heat-treated to a temperature of about1350° C. for a period of about 1 hour by laser heating.

FIG. 8 shows an image taken by the atomic force microscope (AFM) of thesurface of the specimen after the heat treatment. In the Figure,vertical stripes represent atomic surface steps each of whichcorresponds to mono-molecular layer, and a region between the adjacentvertical stripes represents an identical atomic surface. From the Figureit is seen that the surface of Ba_(0.5)Sr_(0.5)TiO₃ epitaxial thin filmis flat on an atomic level. Thus, according to the method of the presentinvention, the dislocations are anchored to the substrate surface, andthe surface of the second epitaxial thin film is flattened to an atomiclevel.

FIG. 9 is a figure illustrating a result of measurement by a four-axisX-ray diffraction apparatus of a distribution of lattice constants inthe specimen mentioned above. In the figure, the abscissa and ordinateaxes are taken in the (300) and (003) directions of a reciprocal latticespace, respectively. In the figure, the point C indicates thediffraction point of the Ba_(0.5)Sr_(0.5)TiO₃ epitaxial thin film as thesecond epitaxial thin film where its diffraction intensity is themaximum, and the point A indicates the diffraction point of the BaTiO₃epitaxial thin film as the first epitaxial thin film where itsdiffraction intensity is the maximum. From the coordinate of point C itis seen that the in-plane lattice constant a of the Ba_(0.5)Sr_(0.5)TiO₃epitaxial thin film is 3.905 angstroms, which is a value as a latticeconstant intermediate between the lattice constant of SrTiO₃ bulkcrystal of the substrate (a=3.905 angstroms) and the lattice constant ofBaTiO₃ bulk crystal (a=4.000 angstroms). From the coordinate of point Ait is seen that the in-plane lattice constant a of the BaTiO₃ epitaxialthin film as the first epitaxial thin film is 3.955 angstroms, which isa value as a lattice constant approximately equal to the in-planelattice constant of Ba_(0.5)Sr_(0.5)TiO₃ mentioned above. The fact thatthe film thickness of Ba_(0.5)Sr_(0.5)TiO₃ being 1800 angstroms is muchthicker than the film thickness of BaTiO₃ being 120 angstroms allows thein-plane lattice constant of the BaTiO₃ epitaxial thin film as the firstepitaxial thin film to take a value close to the in-plane latticeconstant of Ba_(0.5)Sr_(0.5)TiO₃.

Changing the ratio in thickness of the first to the second epitaxialgrowth films changes the in-plate lattice constants of the first andsecond epitaxial thin films. It is also possible to adjust the in-planelattice constants by way of only the film thickness of the secondepitaxial thin film.

While in this specific example the ratio x of components is made 0.5, itwill be apparent that suitably changing the ratio x allows realizing adesired in-plane lattice constant intermediate between the lattice ofsubstrate SrTiO₃ (a=3.952 angstroms) and the lattice constant of bulkBaTiO₃ (a=4.000). According to the method of the present invention it isthus seen that suitably selecting the ratio of component x allowsadjusting the in-plane lattice constant of a second epitaxial thin filmto have a desired value.

While in the foregoing description the heat-treatment temperature isshown to be lower than the melting points of materials used, somematerials have constituent atoms diffusing in their solid state. Withsuch materials, it is desirable that the heat treatment be effected at atemperature lower than their solid-state diffusion startingtemperatures, namely their sintering temperatures.

Also, while in the foregoing specific examples, an example is taken ofoxides having a perovskite-type crystallographic structure, it should beapparent that the oxides may be those having a differentcrystallographic structure, e. g., of a hexagonal system. It should alsobe apparent that the materials to which the present invention isapplicable are not limited to oxides and may be any other materials.

INDUSTRIAL APPLICABILITY

As will be appreciated from the foregoing description, the presentinvention makes it possible to adjust the in-plane lattice constant of asubstrate to have a desired value. Accordingly, the use of an in-planelattice constant adjusted substrate of the present invention for adevice utilizing an epitaxial thin film allows the device to be preparedhaving an extremely high quality. The present invention is extremelyuseful when used as a substrate, e. g., for high temperature oxidesuperconducting device.

1. A method of preparing an in-plane lattice constant adjustedsubstrate, characterized in that it comprises the steps of: forming at afirst preestablished temperature on a single crystal substrate whosesurface is flat on an atomic level, a first epitaxial thin film made ofa first material that is different from a material of which thesubstrate is made; and heat-treating at a second preestablishedtemperature the substrate having the first epitaxial thin film formedthereon, wherein said first preestablished temperature is a temperaturethat causes said first epitaxial thin film to epitaxially grow on saidsubstrate; and said second preestablished temperature is a temperaturethat is higher than said first preestablished temperature but lower thanthe lower of melting points of said substrate and said first epitaxialthin film, whereby the heat treatment at the second preestablishedtemperature gives rises to a modification of said substrate such thatdislocations are introduced into an interface between the substrate andthe first epitaxial thin film whereby the in-plane lattice constant ofthe first epitaxial thin film is altered to have a value that is closeto a bulk lattice constant of said first material, and such that the topsurface of said first epitaxial thin film is flattened to an atomiclevel.
 2. A method of preparing an in-plane lattice constant adjustedsubstrate as set forth in claim 1, characterized in that said substrateand said first epitaxial thin film are made of oxides.
 3. A method ofpreparing an in-plane lattice constant adjusted substrate as set forthin claim 1, characterized in that said substrate is a SrTiO₃ crystallinesubstrate and said first epitaxial thin film is made of BaTiO₃.
 4. Amethod of preparing an in-plane lattice constant adjusted substrate,characterized in that it comprises the steps of: forming on a singlecrystal substrate whose surface is flat on an atomic level, a firstepitaxial thin film having a first preselected film thickness and madeof a first material that is different from a material of which thesubstrate is made, and then forming on the first epitaxial thin film, asecond epitaxial thin film having a second preselected film thicknessand made of a second material that contains, at a predetermined ratio ofcomponents, a substance of said first material and another substancewhich is capable of forming a solid solution; and thereafterheat-treating them at a second preestablished temperature that is higherthan an epitaxial growth temperature of said first and second epitaxialthin films but lower than the lowest of melting points of saidsubstrate, said first epitaxial thin film and said second epitaxial thinfilm to introduce dislocations into an interface between said substrateand said first epitaxial thin film and an interface between said firstand second epitaxial thin films, whereby a modification of saidsubstrate ensures having an in-plane lattice constant of said secondepitaxial thin film controllably determined by a ratio of said first tosecond film thickness and/or a said predetermined ratio of componentsand having the top surface of said second epitaxial thin film flattenedto an atomic level.
 5. A method of preparing an in-plane latticeconstant adjusted substrate as set forth in claim 4, characterized inthat said second epitaxial thin film is formed having an in-planelattice constant controllably determined by only a ratio of componentsthereof without forming said first epitaxial thin film made of the firstmaterial.
 6. A method of preparing an in-plane lattice constant adjustedsubstrate as set forth in claim 4, characterized in that said substrateand said first and second epitaxial thin films are made of oxides.
 7. Amethod of preparing an in-plane lattice constant adjusted substrate asset forth in claim 4, characterized in that said substrate is a SrTiO₃crystalline substrate, said first epitaxial thin film is made of BaTiO₃and said second epitaxial thin film is made of Ba_(x)Sr_(1−x)TiO₃ where0<x<1.
 8. An in-plane lattice constant adjusted substrate, characterizedin that it comprises a SrTiO₃ crystalline substrate and having a thinfilm of BaTiO₃ formed thereon, wherein the BaTiO₃ thin film has its topsurface flattened to an atomic level and is substantially equal inlattice constant to BaTiO₃ bulk crystal.
 9. An in-plane lattice constantadjusted substrate, characterized in that it comprises a SrTiO₃crystalline substrate and having a thin film of BaTiO₃ formed thereonand a thin film of Ba_(x)Sr_(1−x)TiO₃ (where 0<x<1) formed on the BaTiO₃thin film, wherein the Ba_(x)Sr_(1−x)TiO₃ thin film has its top surfaceflattened to an atomic level and has its in-plate lattice constantadjustable to a desired length between the lattice constants of SrTiO₃and BaTiO₃ bulk crystals by selecting x.